My above-identified patent describes a preferred form of such a switching device in which the control circuit employs a flip-flop (FF) circuit which functions as a bistable latch for controlling the ON/OFF states of a light-emitting diode (LED). The latter, in turn, activates a photo-sensitive device whose conduction state tracks that of the LED. The photo-sensitive device is connected in series with the gate of a triac, whose ON/OFF state in turn depends upon the current drive to its gate. When the triac is ON, the load is connected through the triac across the AC power line. The related patent also describes a novel enclosure for the switch which allows in one embodiment mounting of the switch in a wall junction box and provides a suitable heat transfer path for the triac which carries the full load current.
The present invention is directed to an improvement in the control circuit for the solid state switch to widen its range of possible applications. This will be best understood from a brief recapitulation of one of the circuits disclosed in the patent, which is illustrated in FIG. 1 in schematic, block form, reference being had to the patent for a more detailed description of that circuit.
The circuit of FIG. 1 comprises an AC line voltage source 10 connected to the primary of a step-down transformer 11 whose secondary is connected to a filtered rectifier shown schematically as a capacitor 12 and diode 13 for producing an unregulated direct current (DC) supply voltage labelled V+ for operating the solid state components of the control circuit. The AC line 10 is also connected to a load 14 via the cathode/anode circuit of a triac 15. The triac gate 16 is in series with a current limiting resistor 17 and a photo-sensitive device 18 shown as a diac. The latter is optically coupled to an LED 19. Usually, the LED 19 and photo-sensitive device 18 are packaged and sold together as a single component. When current equal to, for example, 5 mA is sourced to the LED 19, it outputs radiation 20 which turns on device 18 causing a current flow into the triac gate 16 which turns on the triac allowing current from the AC line to flow to the load 14.
The control circuit of FIG. 1 comprises an SR FF 22. The S and R inputs of the FF 22 are buffered by special circuitry to allow the switch to be compatible with an earlier electromechanical version. The buffer circuits 23, 24 are respectively connected to the S and R inputs of the FF 22 and momentary ON 25 and OFF 26 switches. As explained in my related patent, the buffers 23, 24 allow an AC signal voltage to be present at the ON 25 or OFF 26 switches without toggling the SR FF 22 to either of its states, in one of which the LED 19 is activated and in the other of which the LED remains OFF. Only when either switch 25, 26 is closed (and connected to ground potential) for a predetermined interval of time will the buffers 23, 24 allow the "ground" signal to pass through to the R and S inputs of the FF 22 and allow it to change state. So long as AC power is applied, the bistable FF retains the state of the switch determined by which of the ON, OFF switches 25, 26 was last activated.
There are applications in which it is important that the AC power switch retain or remember its current state in the event of loss of the AC line voltage, due, for example, to a temporary blackout or power loss or outage situation. When power is restored, it may be necessary or desirable that the load is restored to the state it was in when power was lost, i.e. powered ON or remain OFF, without it being necessary to re-activate one of the momentary switches 25,26.
U.S. Pat. No. 4,563,592 describes a wall box dimmer switch employing a latching relay, an electro-mechanical device, to preserve the on-off status of the switch if the AC supply fails. In FIG. 3a, an electronic latch using a flip-flop circuit is illustrated. No explanation is given as to how a standard FF component can preserve its state upon power loss. The state that a standard FF will come up in on power being applied is unpredictable, as distinguished from a latching relay which could use special means, such as magnets, to hold contacts closed on power loss.
The use of an electro-mechanical relay may not have the extended lifetime possessed by solid state components. It will be noted that the device is intended to be housed in an electrical junction box, and would be expected to operate without problems for 20-30 years. Electro-mechanical components could not normally be relied on to perform flawlessly for that long period of time. Addition of a back-up battery to maintain a standard FF in a given state will not solve the problem, since batteries, even if continually re-charged, cannot be expected to provide 20-30 years of life. A large capacitor that stores charge can be used temporarily to preserve operating voltage for FF components, but their ability to maintain voltage is limited, especially if current continues to flow in the circuit, gradually dissipating the stored charge. However, one would want a large capacitor for maximum protection, but large capacitors are usually of large size, making it difficult to fit them within the usual electrical junction box. The same problem occurs with an electro-mechanical relay, which may be of a size making it difficult or impossible to fit into a junction box. Moreover, high temperature operating conditions that may occur in an enclosed junction box reduce the life of batteries and capacitors.